The aim of this study is to gain a better understanding of the mechanism of absorption of water-soluble vitamins in mammalian small intestine. The application of new research techniques may provide a more accurate and detailed understanding of the transport processes of several vitamins which have been studied previously. The vitamins whose intestinal absorption we currently have little or no information about will be investigated to characterize the basic transport properties. Absorption of vitamins labeled with 14C or 3H will be investigated by measuring transmural fluxes, cellular accumulation, uptake across the brush border membrane of intact tissue and movement into vesicles made from brush border (and perhaps basolateral) cell membranes. Experiments are designed to evaluate the adequacy of models of transport previously proposed to describe absorption of several vitamins. Thus, the acid microclimate hypothesis for folic acid absorption can be evaluated in additional detail with brush border vesicles which maintain no acid microclimate. Similarly, the suggestions that phosphorylation is a necessary step in cellular uptake of thiamine and other vitamins can be evaluated in the brush border vesicle preparation which lacks phosphorylating ability. Also, the transport properties of specific vitamins will be studied to determine a) if vitamin influx across the intestinal brush border proceeds by a carrier-mediated process, b) if vitamin influx is dependent on either intracellular or extracellular Na, c) if vitamin influx across the brush border membrane is a rheogenic process, which would indicate that the intracellular electrical potential normally contributes to vitamin absorption, d) if there is competition for transport with other vitamins, nutrients or structurally related compounds, e) if vitamins accumulate in the cellular fluid against a concentration gradient, a concentration gradient, f) if net transport of vitamins takes place from mucosa to serosa, and g) if animals on specific high or low vitamin diets have altered intestinal transport properties for that vitamin. More accurate therapy for vitamin malabsorption and deficiency may result from these studies on the pertinent G.I. physiology.